In many industrial processes and applications employing synthetic rubber latex, the ability to obtain a mechanically stable latex composed of specific particle sizes and size distributions is of great importance. For many commercial applications, such as in the manufacture of foam rubber, the higher the latex solids content and the lower the viscosity, the more desirable is the synthetic rubber latex composition. Latices for coating and adhesive applications show optimum properties when their particle size is near 200 nanometers (nm) in diameter. At much above 200 nm, the surface tension forces developed in drying water from the polymer particles is not adequate to produce rapid coalescence into a continuous film. These surface tension forces increase for smaller polymer particle sizes; however, the smaller the sizes, the lower the maximum solids which can be achieved before the latex goes from water-like viscosity to a thick gel over an increase in solids of no more than 1% or 2%.
The larger a latex particle size, the higher the solids it can achieve before becoming too viscous for satisfactory flow. A broad particle size distribution extending from less than one hundred nm to several hundred nm enhances the solids achievable until they approach a 70% solids region. Applications of synthetic rubber latex in foam rubber articles and carpet backing requires high solids content to prevent syneresis. Such articles must coalesce rapidly during drying to optimize cushion characteristics in the finished product.
Latex particles of large particle sizes are also required for other applications, such as production of high-impact strength and hard polymers. If a hard, brittle polymer receives a sharp blow, it will be apt to shatter unless latex particles are randomly dispersed in the middle polymer continuous phase to absorb the energy and stop the crack.
Emulsifiers used to coat the latex particles and maintain the colloidal stability of the emulsion particles in suspension are sometimes fatty acid salts, the soap ions of which are absorbed to the latex particles to produce electrically negative surfaces. These charges attract positive cations (counter ions) into the aqueous region surrounding the particles to neutralize the absorbed soap ions on the latex particles and produce a diffuse electrostatic double layer about each latex particle. If these diffuse double layers are compressed sufficiently close to the particles by a high concentration of counter ions in the system, coagulation of the latex will occur. Thus, without satisfactory electrostatic stabilization, the latex will irreversibly coagulate or "preflock" during handling and storage prior to use. On the other hand, if the concentration of rubber particles within the system is sufficiently great that their electrostatic double layers are forced close to each other, a secondary electroviscous effect occurs causing interaction between neighboring particles which produces a high viscosity gel. Dilution of the system or compression of the double layers somewhat with a little additional electrolyte restores such a latex to its original viscosity and its behavior characteristics.
Latex solids approaching 70% are generally achieved by a blend of about 73.5% by volume fraction of larger particles of the order of 200 nm-250 nm diameter and about 26.5% of small particles one-half or less the size of the large ones. Such a ratio allows a maximum solids compaction or packing before the secondary electroviscous effect occurs.
The existing technology used to produce latexes of specific particle size characteristics and distribution for these and other end-use applications is sophisticated and somewhat costly. The particle size obtained in a latex, in emulsion polymerization, usually ranges from about 40 nm up to about 120 nm in diameter. The emulsifier for the latex and the characteristics of the polymerization system utilized govern the specific size which is achieved. If 200 nm diameter particles are to be produced by agglomeration, many 120 nm particles must coagulate the coalesce together. Sometimes large relatively monodisperse latexes are produced by utilizing the original latex as seed, adding more monomer, sometimes in a semi-continuous manner, and polymerizing to the desired size. More emulsifier is also required to maintain colloidal stability of the larger particles. However, care must be exercised that the new emulsifier never reaches a concentration near its critical micelle concentration in the aqueous phase, otherwise, new particles will be generated to give small particle sizes along with the desired 200 nm particles.
Currently, there are several methods used to commercially agglomerate latices to a desired particle size and broad size distribution required for their end-use applications. One such agglomeration process involves freezing the latex under carefully controlled temperature conditions, after which it is melted, or thawed, and heat concentrated at atmospheric pressure. Such freezing and thawing processes are described in U.S. Pat. Nos. 2,993,020 and 3,296,178.
Another process for producing synthetic rubber latexes of increased particle size involves mechanical agglomeration under precise conditions of shear followed by atmospheric pressure heat concentration.
A third method currently employed to commercially agglomerate and control particle sizes of synthetic rubber latexes involves agglomerating during emulsion polymerization of the latex in a system with only a small amount of emulsifier present. When the latex particles have grown until their surfaces are sufficiently starved of emulsifier for the particles to become colloidally unstable, shear mixing of the latex particles partially agglomerates the same. The latex may be kept from totally coagulating by quickly adding sufficient emulsifier to again produce a colloidally stable system precisely at the moment when the emulsifier should be added to achieve satisfactory product. Sometimes shear conditions are such that agglomeration reduces the particle surface area sufficiently so the original emulsifier present can saturate the particle surfaces and prevent coagulation. The agglomerated latexes may be then further polymerized to produce the final product. Since the rate of polymerization is dependent on the number of rubber particles in the system, polymerization is generally much slower after agglomeration than before. Methods of polymerization agglomeration of latex systems are disclosed in U.S. Pat. Nos. 3,080,334; 3,318,831; and 3,607,807.
Natural rubber latex and synthetic rubber latexes may be creamed, i.e., separated into a rubber rich cream portion and a serum portion poor in rubber, by the addition thereto of a vegetable mucilage, such as an alginate, locust bean gum, Irish moss, and the like. Although the creaming method of concentration works well with natural rubber latex because the particle size of the latex is quite large and because the difference in the density between the rubber and media is about 0.08 gm per cc, synthetic rubber latexes cannot be concentrated readily to high solids content by creaming because the particle size is much smaller and the density difference is also smaller than that of natural rubbers.
U.S. Pat. Nos. 2,444,689 and 2,444,801 disclose methods of increasing the size of synthetic latex particles by treating with salts of certain inorganic monovalent cations, organic amine salts, or organic acids in the proper concentrations. In particular, U.S. Pat. No. 2,444,801 discloses that inorganic salts, such as ammonium, sodium, potassium, and lithium salts, in high concentrations cause agglomeration of rubber particles in synthetic latexes to such an extent as to coagulate the rubber latexes. The patent states that treatment of synthetic rubber latexes containing 20 to 30 percent solids with solutions of the inorganic salts at low concentrations, i.e., at or below 7 (seven) weight percent makes it possible to obtain an enlargement of latex particle size without excessive coagulation, and at the same time, increase the density of the media. Such an increase in particle size is stated to facilitate creaming to concentrated latexes containing approximately 50% total solids or higher.
Although the above-mentioned processes of agglomeration of synthetic rubber latexes are commercially employed, they involve relatively complicated and expensive procedures requiring carefully controlled conditions to produce the end-products desired.